Re: [Qemu-devel] qcow2: online snasphots : internal vs external ?

[Alexandre DERUMIER <aderumier@odiso.com>]

Thanks again Stefan

>>The snapshot only captures the contents of the disk. Rollback does
>>not make sense without shutting down the guest. The OS/file system
>>would be very confused if the disk contents changed underneath it.

>>Existing hotplug can be used. For example, if we have an external
>>snapshot of a virtio-blk drive, we can use hotplug to remove the
>>drive, choose the snapshot file and attach it again. This only works
>>for "data" drives, the root file system usually cannot be changed
>>while the guest is running.


Yes, sure rollback must be done offline.
But I wanted to say, with external snapshot, how can I rollback to the point of the snapshot.

exemple :
image1.qcow2
file : /beforesnap1
take a snaphot (snap1), so qemu switch to snap1.qcow2
write some file:
file:
    /aftersnap1.
    /beforesnap1

Now, how can I rollback to the point of time of snap1 ?
I can reuse image1.qcow2, but if I write some datas on it, I don't see how I can return to the point of time of the snap1. (like qemu-image -a  with internal snapshots)


>>You may also wish to look at libvirt for higher level snapshot primitives.
Thanks, I'll look at the libvirt to see how they do things.


----- Mail original -----

De: "Stefan Hajnoczi" <stefanha@gmail.com>
À: "Alexandre DERUMIER" <aderumier@odiso.com>
Cc: "Jeff Cody" <jcody@redhat.com>, "qemu-devel" <qemu-devel@nongnu.org>, "Paolo Bonzini" <pbonzini@redhat.com>, "Eric Blake" <eblake@redhat.com>
Envoyé: Lundi 27 Août 2012 11:04:14
Objet: Re: [Qemu-devel] qcow2: online snasphots : internal vs external ?

On Sun, Aug 26, 2012 at 10:56 AM, Alexandre DERUMIER
<aderumier@odiso.com> wrote:
> It is possible to achieve the same behaviour with external snapshot ? (I would like to do it online)
> I don't see how I can rollback to the point of time of the snapshot.

The snapshot only captures the contents of the disk. Rollback does
not make sense without shutting down the guest. The OS/file system
would be very confused if the disk contents changed underneath it.

Existing hotplug can be used. For example, if we have an external
snapshot of a virtio-blk drive, we can use hotplug to remove the
drive, choose the snapshot file and attach it again. This only works
for "data" drives, the root file system usually cannot be changed
while the guest is running.

You may also wish to look at libvirt for higher level snapshot primitives. 

> Also I see that snapshot_blkdev qmp command give in his description:
> "Otherwise the snapshot will be internal! (currently unsupported)."
>
> is Live internal snapshots on the roadmap ?

I'm not aware of anyone working on adding internal snapshot in the
near future. Patches are welcome.

Stefan



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Alexandre D e rumier

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